This invention relates to detection of leaks in sealed articles and, more particularly, to methods and apparatus for leak detection at relatively high test port pressures which occur at high leak rates.
Helium mass spectrometer leak detection is a well-known leak detection technique. Helium is used as a tracer gas which passes through the smallest of leaks in a sealed test piece. The helium is then drawn into a leak detection instrument and measured. The quantity of helium corresponds to the leak rate. The most important component of the instrument is a mass spectrometer tube which detects and measures the helium. The input gas is ionized and mass analyzed by the spectrometer tube in order to separate the helium component, which is then measured. In one approach, a test piece is pressurized with helium. A sniffer probe connected to the test port of a leak detector is moved around the exterior of the test piece. In the vicinity of a leak, helium is drawn into the probe and is measured by the leak detector. In an alternative approach, the interior of a test piece is coupled to the test port of the leak detector. Helium is sprayed onto the exterior of the test piece, is drawn inside through a leak and is measured by the leak detector. Helium mass spectrometer leak detection provides good sensitivity, a wide dynamic range, and is fast and convenient to use.
One or the requirements of the spectrometer tube is that the inlet through which the helium and other gases are received be maintained at a relatively low pressure, typically below 2.times.10.sup.-4 torr. In a so-called conventional leak detector, a roughing pump, a diffusion pump and associated forepump, and a cold trap are utilized in a conventional vacuum pumping arrangement to maintain the input of the spectrometer tube at the required pressure. However, since the test port must be maintained at a relatively low pressure during a leak test, the rough pumping cycle is relatively long. Furthermore, in the testing of leaky or large volume parts, which results in a high test port pressure, it may be difficult or impossible to achieve the required pressure level. If the required pressure level can be achieved at all, the pumping cycle is lengthy.
Many of these problems were eliminated in the counterflow leak detector disclosed by Briggs in U.S. Pat. No. 3,690,151. In this leak detector, the mass spectrometer tube is connected to the inlet of a diffusion pump and the helium tracer gas is introduced through the foreline, or normal output port, of the diffusion pump. The diffusion pump exhibits a high pressure ratio for heavier gases, but a low pressure ratio for lighter gases such as helium. Therefore, helium diffuses at an acceptable rate in a reverse direction through the diffusion pump to the spectrometer tube and is measured. Heavier gases in the sample are, to a large degree, blocked by the diffusion pump and prevented from reaching the spectrometer tube. Due to the method of reverse flow in the diffusion pump, the leak detector test port can be operated at the pressure required at the diffusion pump foreline. This pressure, typically 100 millitorr, is several orders of magnitude higher than the required operating pressure of the spectrometer tube and is relatively easy to attain.
A test port pressure of 100 millitorr is satisfactory for many leak test applications. Nonetheless, it is desirable in some applications to perform leak tests on very large or leaky parts where this test port pressure level cannot be attained. Heretofore, it has been possible to perform helium mass spectrometer leak tests with test port pressures above 100 millitorr, but only with the use of two mechanical vacuum pumps and an intermediate flow restrictor to maintain the foreline pressure at or below 100 millitorr.
It is a general object of the present invention to provide novel methods and apparatus for mass spectrometer leak detection.
It is another object of the present invention to provide methods and apparatus for performing mass spectrometer leak tests at test port pressures substantially above 100 millitorr utilizing only one mechanical pump.
It is still another object of the present invention to provide methods and apparatus for leak detection wherein the helium tracer gas passes in reverse direction through at least one stage of a mechanical vacuum pump, so as to permit testing at high test port pressures.